The load bearing capacity of the soil supporting highways, airfield runways and other pavement systems is of immense importance to the integrity of the pavement. This load-bearing capacity, or soil stiffness, changes from time to time and can vary from place to place within a given area.
Soil stiffness is the degree of resistance to deformation upon loading. The extent and time-dependence of, and the degree of recovery from, deformation is primarily dependent upon the soil's properties, existing stress conditions, and the stress history. Soil properties in turn are determined by a variety of complex interrelated factors, including composition particle size and particle-size distribution, weight-volume relationships, and in-situ stresses.
The stability or load-bearing capacity (capability) of the pavement of airport runways, highways and other pavement systems is determined in significant part by the load-bearing capacity of the underlying subpavement) earth or soil, which may deteriorate over time due to environmental and stress influences on soil properties. For instance, changes in soil load-bearing conditions due to changes in moisture content and/or repeated loading over time are well recognized in engineering fields. In addition, certain pavement systems such as runways and highways typically endure repeated severe loadings on a daily basis.
The proper determination of existing bearing-load capacities of soil-supported pavement systems requires that the existing soil conditions be defined and evaluated. Conventional soil-structure modeling is based on the results of laboratory testing of individual localized soil samples, as in the case of the well-known California Bearing Ratio, or CBR, laboratory test. However, tests such as the CBR are severely disadvantaged because the test conditions and the soil sample (specimen) are not representative of in-situ conditions. Absent are (a) in-situ overburden stress, (b) in-situ soil interactions, and the like. Further, many if not most soil samples have been disturbed to some degree during sampling and handling. A true composite soil stiffness determination can only be determined using actual stiffness data of in-situ soil conditions at varying depths (varying subgrade conditions). In addition, while soil samples from individual lifts of soil placement can be obtained with relative ease before and/or during construction of the pavement system, thereafter the overlying structure generally precludes sampling of the supporting soil by nondestructive methods.
Another known method for determining composite soil stiffness is the use of plate bearing tests on the surface of soil layers.
As mentioned herein above, the current most widely used way to determine soil stiffness is by using the California Bearing Ratio (CBR) test on soil samples that are prepared in the laboratory, the objective being to calculate with the stiffness, or resilient modulus of soils, MR, using generally accepted empirical expressions. An example of such an expression is the one recommended by American Association of State Highway and Transportation Officials (AASHTO):MR=10,340CBR(kPa)  (1)
The limitations of the CBR are discussed hereinabove. In summary they derive from that fact that the soil specimens of base, subbase, or subgrade layers are prepared in the laboratory using compaction procedures that are different from what the soils are subjected to in the field. Therefore, laboratory specimens may have stiffnesses that are different from the soils compacted in the field. Also, the CBR test does not provide the information needed by engineers to determine whether the stiffness of subgrade, subbase, and base soils of a pavement under construction in the field meets the design requirement, and for an existing pavement where the subgrade, subbase, and the base soils have gone through years of weather cycles and traffic load applications, the CBR test by itself cannot provide a realistic or representative measurement of soil stiffness. Therefore, there is the need to develop an in-situ, non-destructive, accurate and economical method for the measurement of stiffness of soils for pavement engineering applications.
Generally, soil stiffness determinations require (a) the application of a predetermined surface force and (b) the measurement of the resultant deflection or vertical deformation of the soil. Apparatus for applying a predetermined surface force are well known. Apparatus for measuring resultant deflection at the surface are also known. The challenge is the instrumentation and methodology needed to obtain actual stiffness data of in-situ soil conditions at varying depths to obtain the data necessary for the definition and evaluation of existing soil conditions, and then to properly determine existing bearing-load capacities of the overlying pavement system.
The most direct method of measuring composite and individual soil layer stiffness and deflections is through the use of a multi-depth deflectometer (“MDD”). U.S. Pat. No. 6,386,044 to Weinmann is an example of an MMD designed to be installed in a borehole for long periods during which data on soil layer displacements cane be gathered and stored for analysis.